Thermal and catalytic conversion of hydrocarbons to olefins is an important industrial process producing millions of pounds of olefins each year. Because of the large volume of production, small improvements in operating efficiency translate into significant profits. Catalysts play an important role in more selective conversion of hydrocarbons to olefins.
Particularly important catalysts are found among the natural and synthetic zeolites. Zeolites are crystalline aluminosilicates with a network of AlO.sub.4 and SiO.sub.4 tetrahedra linked by oxygen atoms. The negative charge of the network is balanced by the inclusion of protons or cations such as alkali or alkaline earth metal ions. The interstitial spaces or channels formed by the crystalline network enable zeolites to be used as molecular sieves in separation processes and in catalysis. There are a large number of both natural and synthetic zeolitic structures including materials with additional elements such as boron, iron, gallium and titanium. The wide breadth of zeolite structures is illustrated in the "Atlas of Zeolite Structure Types" by W. M. Meier, D. H. Olson and Ch. Baerlocher (4th ed., Elsevier/Intl. Zeolite Assoc. (1996)). Catalysts containing zeolites, especially medium pore zeolites, are known to be active in cracking light naphtha to light olefins, primarily propylene and butylenes, as well as heavier hydrocarbon streams.
Of particular interest are the proton form zeolites effective for conversion of hydrocarbons such as naphthas to olefins. Typical catalysts include ZSM-5 zeolite described and claimed in U.S. Pat. No. 3,702,886, and ZSM-11 described in U.S. Pat. No. 3,709,979, and the numerous variations on these catalysts disclosed and claimed in later patents. It has previously been observed that treatment or addition of either phosphorus or aluminum phosphate separately produced a small improvement in certain zeolite catalysts. U.S. Pat. No. 4,605,637 teaches activation of low acidity zeolite with an aluminum phosphate in contact with an aqueous liquid phase. U.S. Pat. No. 4,977,122 teaches use of phosphorus containing alumina to increase the activity of catalysts. U.S. Pat. No. 5,378,670 discloses use of phosphorus compounds to activate zeolites. U.S. Pat. No. 5,457,078 teaches a method for manufacture of improved zeolite beta catalyst wherein a matrix substantially free of crystalline aluminum phosphate is generated by treating the zeolite and a source of acid soluble source of aluminum such as pseudoboehmite with a phosphorus compound such as phosphoric acid. The resulting mixture is spray dried or extruded and may optionally be calcined. In one example ZSM-5 is used. However the composition is specifically described and claimed as a matrix that is substantially free of crystalline aluminum phosphate.
The art has not previously recognized the synergistic effect of phosphorus and aluminum phosphate in combination with small and medium pore acidic zeolites. In contrast to the teaching of U.S. Pat. No. 5,457,078, which taught the use of a catalyst substantially free of crystalline aluminum phosphate, the present inventors found that a combination including aluminum phosphate and phosphorus treated medium and small pore acidic zeolites provide a synergistically improved cracking catalyst with improvements significantly larger than the sum of improvements noted with either component alone.